226 CLAYS AND CLAY INDUSTRY. 



charged at one end, and the blades on the shaft not only cut up the 

 clay lumps, but mix the mass, at the same time pushing it towards 

 the discharge end. The speed of the clay through the machine 

 varies with the angle of the blades on the shaft. It will be seen 

 at once that the thoroughness of the mixing depends to a large 

 degree on the length of the pug mill, as well as the angle at which 

 the blades are set. For some clays, therefore, it is very inadvis- 

 able to' use a short mill, one not more than 3 or 4 feet long. 



Pug mills are thorough and continuous in their action, take 

 up less space than ring pits and do< not require much power to 

 operate. They are nearly always used in connection with stiff- 

 mud machines, and at the present day are often used with the 

 soft-mud process. 



Wet pans. — These are similar to> dry pans, but differ from 

 them in having a solid bottom. The material and water are put 

 into the pan and the clay is crushed and tempered at the same 

 time. Where the clay contains hard lumps of limonite or pyrite 

 nodules, a wet pan is superior to a pug mill or disintegrator, for 

 the charge is crushed and tempered in a few minutes, and can 

 then be replaced by another one. 



MOLDING. 



Bricks are molded by one of four methods, viz., soft-mud,, 

 stiff-mud, dry-press, and semidry-press, although in reality there 

 is not much difference between the last two methods. 



Soft-mud process. — In this method the clay, or clay and sand, 

 are mixed with water to the consistency of a soft mud or paste 

 and pressed into wooden molds. Since, however, the wet clay 

 is sticky and likely to adhere to a wooden surface, the molds 

 are sanded each time before being filled. Soft-mud bricks, 

 therefore, show five sanded surfaces, and the sixth surface will 

 be somewhat rough, due to- the excess of clay being wiped off 

 even with the top of the mold. They are also< slightly convex on 

 one side and slightly concave on the other, due to- the sides of the 

 soft brick dragging slightly as it is dumped from the mold to 

 the drying floor. 



Soft-mud bricks are molded either by hand or in machines. 



